Measure battery condition (safety critical unit)?

I'm maybe going to build a battery powered embedded unit. It would be necessary to determine the battery's condition at power on to alert the user that he must change the battery.

The unit must know that it can activate a relay at least once during power on. That is the purpose of the battery test.

The unit is going to have a microcontroller.

Any ideas of how to make that test? Maby measure the battery voltage when the unit applies a resistive load to it. And then determine the V and the Ri of the battery.

Should I use two batteries so I can measure one at a time at some interval?

It's for a safety critical unit.

Reply to
Mr M
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That is the usual way of doing it. It may not work very well for a device that's going to be stored for a long time with the battery disconnected; you need to choose the right battery chemistry for the application. (Sounds like you have to solve some of the same problems as an automatic [parachute] activation device; take a look at how the pros do it in that application)

"critical" voltage should be well above the brownout voltage for the circuit, so you can simply turn on the load and see how far Vbat dips.

Reply to
larwe

Have you considered adding a capacitor (maybe an ultracap)? If it's a short term draw then the cap may allow you to operate with a much lower peak power draw from the battery.

Robert

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Reply to
Robert Adsett

If your system is really safety critical you should consider a complete re-think so that you do not need an active event to achieve a safe state. Obviously 'safety critical' covers a multitude of possibilities but if there is a significant risk of causing death or serious injury I would not advise you to rely on a complex loop involving a battery test and a decision by your software.

Michael Kellett

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Reply to
MK

For the application I described, it is SOP for the unit to do a self- test when the user powers it up (before they step in the plane) and report any problems at that time. It doesn't do the test immediately before the critical event; it does the test when you indicate you will soon be relying on it.

Reply to
larwe

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